Selina Concise Solutions for ICSE Class 8 Chemistry Chapter 4 Atomic Structure

Points to Remember:

• Anything which occupies space and has a mass is called matter.
• Smallest particle of matter is called the atom.
• In 1808 John Dalton, an English Scientist, described the atom as the smallest particle exhibiting all the properties of a particular element. According to him, atom could not be divided further into smaller particles.
• Later discoveries led to modifications of Dalton’s Theory and now is known that an atom consists of three-fundamental particles; electrons, protons, neutrons.
• An atom is electrically neutral i.e. The number of protons in an atom equal to the number of electrons in it.
• The distribution of electrons in the various orbits of an element is called the electronic configuration of that element. The maximum number of electrons in a shell is given by the formula 2n². Where n is the serial number of the shell. But the outer most shell cannot have more than 8 electrons.
• The noble gases do not react chemically because they have a stable configuration of 8 electrons in their outermost orbits.
• Elements are chemically reactive because they can gain or loose electrons to attain stable electronic configuration.
• The number of electrons donated or accepted by an atom during chemical combination is called its valency.
• The charged atoms formed due to the transfer of electrons are called ions. The positively charges ion is called a cation negatively charges ion is called an anion.
• The atoms of an element having the same atomic number but different mass number are called the isotopes of element.
• The number of protons in the nucleus of an atom is called the atomic number, which is equal to the number of electrons in a neutral atom.
• The- sum of the number of protons and the number of neutrons in the nucleus of an atom is called its mass number.

Exercise

Question 1: Fill in the blanks.
(a) Dalton said that atoms could not be divided
(b) An ion which has a positive charge is called a cation.
(c) The outermost shell of an atom is known as valence shell.
(d) The nucleus of an atom is very hard and dense.
(e) Neutrons are neutral particles having mass equal to that of protons.
(f) Isotopes are the atoms of an element having the same atomic number but a different mass number.
Answer:
These blanks cover the basic vocabulary of atomic science. Understanding the names of these parts and theories helps us describe how everything in the universe is built.
Teacher's Tip: Remember "Cation" has a 't' which looks like a plus sign (+) for positive!
Exam Tip: Always double-check the spelling of scientific terms like 'valence' and 'isotopes' to ensure full marks.

Question 2: Write ‘true’ or ‘false’ for the following statements:
(a) An atom on the whole has a positive charge.
(b) The maximum number of electrons in the first shell can be 8.
(c) The central pad of the atom is called nucleus.
Answer:
(a) false (b) false (c) True.
Atoms are neutral because the positive protons and negative electrons balance each other out. The first shell is quite small and can only hold up to 2 electrons, not 8.
Teacher's Tip: Use the 2n² rule; for the first shell, n=1, so 2 × 1² = 2.
Exam Tip: When a statement is false, try to mentally correct it so you truly understand the concept.

Question 3: Give the following a suitable word/phrase.
(a) The sub-atomic particle with negative charge and negligible mass.
(b) Protons and neutrons present in the nucleus.
(c) The electrons present in the outermost shell.
(d) Arrangement of electrons in the shells of an atom.
(e) The number of protons present in the nucleus of an atom.
(f) The sum of the number of protons and neutrons of an atom.
(g) Atoms of same element with same atomic number but a different mass number.
(h) The smallest unit of an element which takes part in a chemical reaction.
Answer:
(a) Neutron (Note: Textbook answer says Neutron, but strictly (a) describes an Electron based on "negative charge")
(b) Mass number
(c) Valency
(d) Orbits or Valence shells
(e) Atomic number
(f) Mass number
(g) Isotopes
(h) Atom
These one-word answers help define the fundamental identities and counts within an atom. Knowing the difference between atomic number and mass number is essential for solving chemical problems.
Teacher's Tip: Atomic number = ID card (only protons); Mass number = weight (protons + neutrons).
Exam Tip: Be careful with definitions; "electrons in the outermost shell" are valence electrons, while the shell itself is the valence shell.

Question 4: Multiple Choice Questions
 

(a) The outermost shell of an atom is known as:
1. valency
2. valence electrons
3. nucleus
4. valence shell
Answer:  4. valence shell

(b) The number of valence electrons present in magnesium is:
1. two
2. three
3. four
4. five
Answer: 1. two

(c) The sub atomic particle with negative charge is:
1. proton
2. neutron
3. electron
4. nucleon
Answer: 3. electron

(d) If the atomic number of an atom is 17 and mass number is 35 then number of neutron will be:
1. 35
2. 17
3. 18
4. 52
Answer: 3. 18

(e) The number of electrons in an atom is equal to number of:
1. protons in a neutral atom
2. neutrons in a neutral atom
3. nucleons in a neutral atom
4. none of the above
Answer: 1. protons in a neutral atom

(f) The sum of number of protons and number of neutrons present in the nucleus of an atom is called its:
1. mass number
2. atomic number
3. number of electrons
4. all of the above
Answer: 1. mass number

MCQs test your ability to quickly identify the correct scientific property. For example, calculating neutrons requires subtracting the atomic number from the mass number (35 - 17 = 18).
Teacher's Tip: To find neutrons, think: Mass - Atomic Number = Neutrons.
Exam Tip: Read all options carefully; sometimes multiple terms sound similar but have different meanings.

Question 5: Name three fundamental particles of the atom. Give the symbol with charge, on each particle.
Answer:
The fundamental particles of the atom are: electrons, protons and neutrons.
Particle: electron Symbol: e^- Charge: -1 or 1.602 × 10^-19 C. Where -1 represent its one unit negative electrical charge
Particle: proton Symbol: p⁺ Charge: + 1 or 1.602 × 10^-19 C. Where +1 represents one unit +ve electrical charge.
Particle: neutron Symbol: n⁰ Charge: 0
Protons and neutrons sit in the center (nucleus), while electrons spin around the outside. The positive charge of protons is exactly balanced by the negative charge of electrons in a normal atom.
Teacher's Tip: Proton is Positive (P for P), Electron is negative, Neutron is Neutral (N for N).
Exam Tip: When asked for symbols, always include the superscript signs like + or - to indicate charge.

Question 6: Define the following terms: (a) Atomic number (b) Mass number (c) Nucleons (d) Valence shell
Answer:
(a) Atomic number: Atomic number refers to the number of protons present in an atom. It is denoted by Z. Example: An atom of oxygen contains 8 proton Therefore its atomic number is 8.
(b) Mass number: Mass number refers to the sum of the number of protons and neutrons present in the nucleus of an atom and denoted by A Mass number = Number of protons + Number of neutrons.
(c) Nucleons: The protons and neutrons collectively are known as nucleons.
(d) Valence Shell: The outermost shell of an atom is known as its valence shell.
Atomic number tells us what the element is, while mass number tells us how heavy it is. Nucleons and valence shell are terms used to describe the location and grouping of these parts.
Teacher's Tip: Think of "Nucleons" as the "residents" who live inside the Nucleus.
Exam Tip: Use the notation A for mass number and Z for atomic number to demonstrate advanced knowledge.

Question 7: Mention briefly the salient features of Dalton’s atomic theory (five points).
Answer: Salient features of Dalton’s atomic theory:
1. Matter consists of very small and indivisible particles called atoms, which can neither be created nor can be destroyed.
2. The atoms of an element are alike in all respects i.e. size, mass, density, chemical properties but they differ from the atoms of other elements.
3. Atoms of an element combine in small numbers to form molecules of the element.
4. Atoms of one element combine with atoms of another element in simple whole number ratio to form molecules of compounds.
5. Atoms are the smallest units of matter that can take part in a chemical reaction during which only rearrangement of atoms takes place.
Dalton's theory was the first scientific step toward understanding how everything is made. Although we now know atoms can be divided into subatomic particles, his rules about combinations still help us today.
Teacher's Tip: Remember the phrase "Atoms are building blocks" to recall his main points.
Exam Tip: State the year (1808) and Dalton's full name to add weight to your descriptive answers.

Question 8: (a) What are the two main features of Rutherford’s atomic model? (b) State its one drawback.
Answer:
(a) According to Rutherford’s model an atom consists of:
1. The centrally located nucleus: The nucleus is a centrally located positively charged mass. The entire mass of the atom is concentrated in it. It is the densest part of the atom. Its size is very small as compare to the atom as a whole.
2. The outer circular orbits: Electrons revolve in circular orbits (shell) in the space available around the nucleus. An atom is electrically neutral i.e., number of protons and electrons present in an atom are equal.
(b) Rutherford’s atomic model could not explain the stability of the atom as it is like a solar system, the sun is at the centre and the planets revolve around it, in an atom the electrons revolve around the centrally located nucleus containing protons.
Rutherford discovered that atoms are mostly empty space with a tiny, heavy core. His "solar system" model was brilliant but couldn't explain why spinning electrons don't just crash into the center.
Teacher's Tip: Think of Rutherford's model like planets orbiting the sun.
Exam Tip: Clearly separate "features" and "drawbacks" with headings to score full marks.

Question 9: What are the observations of the experiment done by Rutherford in order to determine the structure of an atom?
Answer: Following were the observations made by Rutherford:
1. Most of the alpha particle passed straight through the foil without any deflection from their path.
2. A small fraction of them were deflected from their original path by small angles.
3. Only a few particles bounced back.
Rutherford shot tiny particles at gold foil and was shocked that most went straight through. This proved that atoms aren't solid balls but have huge gaps inside them.
Teacher's Tip: Use the "Gold Foil" analogy - shooting bullets through a balloon!
Exam Tip: List observations in the exact order they occurred to show a logical flow.

Question 10: State the mass number, the atomic number, number of neutrons and electronic configuration of the following atoms. Also, draw atomic diagrams for them.
6¹²C, 8¹⁶O, 9¹⁹F, 10²⁰Ne, 13²⁷Al, 17³⁵Cl
Answer:
Name of elements: Carbon (₆¹²C); Atomic number: 6; Atomic Mass: 12; No. of proton: 6; No. of electrons: 6; No. of neutrons: 6; Electronic configuration: 2, 4
Name of elements: Oxygen ({₈¹⁶O); Atomic number: 8; Atomic Mass: 16; No. of proton: 8; No. of electrons: 8; No. of neutrons: 8; Electronic configuration: 2, 6
Name of elements: Fluorine (₉¹⁹F); Atomic number: 9; Atomic Mass: 19; No. of proton: 9; No. of electrons: 9; No. of neutrons: 10; Electronic configuration: 2, 7
Name of elements: Neon (₁₀²⁰Ne); Atomic number: 10; Atomic Mass: 20; No. of proton: 10; No. of electrons: 10; No. of neutrons: 10; Electronic configuration: 2, 8
Name of elements: Aluminium (₁₃²⁷Al); Atomic number: 13; Atomic Mass: 27; No. of proton: 13; No. of electrons: 13; No. of neutrons: 14; Electronic configuration: 2, 8, 3
Name of elements: Chlorine (₁₇³⁵Cl); Atomic number: 17; Atomic Mass: 35; No. of proton: 17; No. of electrons: 17; No. of neutrons: 18; Electronic configuration: 2, 8, 7
(Atomic diagrams showing protons/neutrons in nucleus and electrons in shells like 2,4 or 2,8,3).
This table organizes the "blueprint" for each atom. Electronic configuration shows us how the electrons are parked in different layers around the nucleus.
Teacher's Tip: The first shell (K) always fills with 2 electrons first before moving to the next layer.
Exam Tip: When drawing diagrams, clearly label the number of p and n inside the nucleus circle.

Question 11: What is variable valency? Name two elements having variable valency and state their valencies.
Answer:
Variable valency: Some elements exhibit more than one valency. They are said to have variable valency, e.g. Iron, copper, tin, lead.
Iron Fe: Fe²⁺ or Fe³⁺
Copper Cu: Cu⁺ or Cu²⁺
Some atoms are flexible and can use different numbers of electrons depending on who they are bonding with. This is common in metals like Iron and Copper, which can have multiple "combining powers."
Teacher's Tip: Think of it as a person who can hold one hand or two hands at different times.
Exam Tip: Always provide the specific symbols like Fe²⁺ and Fe³⁺to get full marks for variable valency questions.

Question 12: The atomic number and the mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement.
Answer:
Atomic number = 11; No of protons = 11
Mass number = 23 = Number of protons + Number of neutrons.
No of neutrons = 23 - 11 = 12.
This statement tells us exactly what's inside a sodium atom. We know it has 11 positive protons, 11 negative electrons (to stay neutral), and 12 neutral neutrons in its core.
Teacher's Tip: The Atomic Number is the "Identity" - if it's 11, it's Sodium!
Exam Tip: Break down your answer to show the calculation for neutrons to ensure full points.

Question 13: Draw the diagrams representing the atomic structures of the following: (a) Nitrogen (b) Neon
Answer:
(a) Nitrogen (₇¹⁴N): Nucleus with 7p, 7n. Electronic configuration: 2, 5
(b) Neon (₁₀²⁰Ne): Nucleus with 10p, 10n. Electronic configuration: 2, 8
These diagrams show how the particles are physically arranged. Nitrogen has a partially full outer shell, while Neon has a perfectly full outer shell (8 electrons), making it stable.
Teacher's Tip: Neon is a "Noble" gas because its outer shell is full and it doesn't want to react.
Exam Tip: Make sure your electron dots are clearly visible on the shell circles.

Question 14: Explain the rule with example according to which electrons are filled in various energy levels,
Answer:
The maximum number of electrons that can be present in any shell or orbit of an atom is given by the formula 2n², where n is the serial number of the shell.
Therefore:
K shell, n = 1, no. of electrons = 2 × 1² = 2
L shell, n = 2, no. of electrons = 2 × 2² = 8
M shell, n = 3, no. of electrons = 2 × 3² = 18
N shell, n = 4, no. of electrons = 2 × 4² = 32
Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a stepwise manner.
Example: Helium (Atomic 2) -> K:2; Boron (Atomic 5) -> K:2, L:3; Oxygen (Atomic 8) -> K:2, L:6; Neon (Atomic 10) -> K:2, L:8.
This 2n² rule acts as the seating chart for an atom. Electrons always fill the closest, lowest-energy spots first before moving further away.
Teacher's Tip: It's like filling seats in a theater from the front row to the back row.
Exam Tip: Always show the calculation for 2n² when explaining the rule.

Question 15: The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What is its atomic number and mass number?
Answer:
Protons = 4, neutrons = 5, electrons = 4
Atomic number = 4,
Mass number = 4 + 5 = 9
By counting the protons, we find the atomic number. By adding protons and neutrons together, we find the mass (weight) of the atom.
Teacher's Tip: Protons determine the identity; atomic number 4 belongs to Beryllium.
Exam Tip: Clearly state the formula (Protons + Neutrons = Mass Number) in your answer.

Question 16: (a) What are the two main parts of which an atom is made of? (b) Where is the nucleus of an atom situated? (c) What are orbits or shells of an atom?
Answer:
(a) 1. The centrally located nucleus 2. The outer circular orbits.
(b) The nucleus is a centrally located positively charged mass.
(c) The circular orbits (shell present) in the space available around the nucleus on which electrons revolve are called orbits or shells of an atom.
An atom is like a tiny solar system with a heavy center and light spinning parts. Most of the atom's weight is packed into that tiny center nucleus.
Teacher's Tip: Orbits are the "roads" electrons take around the nucleus center.
Exam Tip: Use descriptive words like "centrally located" and "circular" to define parts clearly.

Question 17: What are isotopes? How does the existence of isotopes contradict Dalton’s atomic theory?
Answer: Atoms of an element must have the same atomic number, but their mass number can be different due to the presence of different number of neutrons. These atoms of an element having different number of neutrons are called isotopes (groups).
According to Dalton’s theory, all atoms of an element are similar to all respects, for example, they have the same shape, size etc. and have similar physical and chemical properties like mass, density and reactivity. Whereas isotopes of an element have atoms that are similar as they have same number of protons and electrons but differ in the number of neutrons. So, the isotopes have atoms that are not similar in all aspects.
Isotopes prove that atoms of the same element don't all weigh the same. Dalton thought all atoms of one element were identical twins, but isotopes show they are more like siblings with different weights.
Teacher's Tip: Isotopes are "Same Element, Different Weight."
Exam Tip: Mention the difference in "neutrons" specifically when explaining why isotopes contradict Dalton.

Question 18: Complete the table below by identifying A, B, C, D, E and F.
Answer: Fluorine: Symbol ₉F¹⁹, protons 9, neutrons 10 (A), electrons 9 (B)
Aluminium: Symbol ₁₃Al²⁷ (C), protons 13 (D), neutrons 14, electrons 13
Potassium: Symbol ₁₉K³⁹, protons 19 (E), neutrons 20 (F), electrons 19
This table exercise helps you link symbols to subatomic particle counts. Remember that protons and electrons are equal in neutral atoms, and neutrons = Mass - Protons.
Teacher's Tip: Check your math! 13 + 14 = 27 for Aluminium.
Exam Tip: Fill in tables neatly and double-check your subtraction for neutrons.

EXTRA PRACTICE QUESTIONS

FILL IN THE BLANKS

1. Atomic theory of matter was given by Democritus.
2. Gases become good conductors of electricity under low (low/high) pressure and high (low/high) voltage electrical discharge.
3. Rutherford won the Nobel Prize in 1908 for his investigations in the field of atomic theory.
4. The electrons revolving in shells close to the nucleus are called bound electrons.
5. In an atom, the last shell containing electrons is called its valence shell.
6. The protons and neutrons are held firmly in the nucleus by strong nuclear forces.
7. Amount of energy stored inside the nucleus is known as nuclear energy.
8. The gamma-rays are electromagnetic radiations, like light.
9. Nuclear energy can be used to make electricity.
Answer: 
These facts expand our knowledge of history and applications of atomic science. For example, knowing that "nuclear forces" hold the core together explains why atoms don't just fall apart.
Teacher's Tip: "Bound" means "stuck" - bound electrons are stuck close to the positive nucleus.
Exam Tip: Remember that Rutherford's Nobel Prize was for "investigations" into radioactivity and atomic structure.

MULTIPLE CHOICE QUESTIONS

1. Which of these have similar chemical properties but different set of physical properties and mass number?
1. isobars
2. elements
3. isotopes
4. none of these
Answer: 3. isotopes

2. Which one of the following scientists put forward the theory regarding the extra-nuclear structure of the atom?
1. James Chadwick
2. John Dalton
3. Rutherford
4. Niels Bohr
Answer: 4. Niels Bohr

3. Which of the following scientists observed that cathode rays consists of negatively charged particles?
1. John Dalton
2. J J Thomson
3. James Chadwick
4. Democritus
Answer: 2. J J Thomson

4. In 1908, R A Millikan found the charge of an electron to be:
1. 1.6 × 10^-19 C
2. 1.6 × 10^-15 C
3. 1.4 × 10^-19 C
4. 1.6 × 19^-16 C.
Answer: 1. 1.6 × 10^-19 C

5. How many times is a proton heavier than an electron?
1. 1500
2. 1800
3. 1840
4. none of these
Answer: 3. 1840

6. Who discovered that radioactive rays are given out by uranium?
1. Henri Becquerel
2. Marie Curie
3. Pierre Curie
4. Schmidt
Answer: 1. Henri Becquerel

7. beta-particles are actually a stream of:
1. protons
2. electrons
3. neutrons
4. none of these
Answer: 2. electrons

8. Which of the following is used as a coolant in nuclear reactor?
1. liquid sodium
2. liquid nitrogen
3. cadmium steel rods
4. plutonium
Answer: 1. liquid sodium
 

MATCH THE COLUMNS

1. John Dalton
2. Atom
3. Chadwick
4. Rutherford
5. Protons
6. Nuclear fusion
7. Gamma-rays
a. Neutrons
b. Model of the atom
c. The Atomic Theory of Matter
d. Positively charged
e. Electrically neutral
f. Generator of electricity
g. High penetrating power
h. Energy in sun
Answer:
1. John Dalton - c. The Atomic Theory of Matter
2. Atom - e. Electrically neutral
3. Chadwick - a. Neutrons
4. Rutherford - b. Model of the atom
5. Protons - d. Positively charged
6. Nuclear fusion - h. Energy in sun
7. Gamma-rays - g. High penetrating power

Matching helps connect names to their important contributions. It's fascinating that the sun's energy comes from the same "nuclear fusion" we study in tiny atoms.
Teacher's Tip: Match the charges (Proton = +) first to clear the easy ones.
Exam Tip: In matching questions, draw straight lines or write the letters clearly next to the numbers.

TRUE \ FALSE

1. Elements combine in definite proportions to form compounds.
Answer: True.

2. Elements having a tendency to accept electrons are called metals.
Answer: False. Element having a tendency to release/donate electrons are called metals.

3. Electrons are present in the nucleus.
Answer: False. Electrons are present in the shells.

4. Neils Bohr gave the theory to explain the arrangement of protons in an atom.
Answer: False. Neils Bohr gave the theory to explain the arrangement of electrons in an atom.

5. Cathode rays are a stream of fast moving particles that travel in random directions.
Answer: False. Cathode rays are a stream of fast moving particles that travel in straight line.

6. Uranium-235 and uranium-238 both undergo nuclear fission.
Answer: True.

7. After the emission of a beta-particle, the atomic number of the atom increases by one.
Answer: True.

These statements clarify the specific roles of subatomic particles and the behavior of rays. For instance, metals are "givers" (donate electrons), while non-metals are "takers" (accept electrons).
Teacher's Tip: Nucleus is the core; Electrons are the outer "cloud."
Exam Tip: Always check if the statement says "increase" or "decrease" in atomic number during decay.

WRITE SHORT ANSWERS

Question 1: What is an atom?
Answer: An atom is defined as the smallest particle of a molecule which is further subdivided into electrons, protons and neutrons.
It is the basic building block of all matter. Every solid, liquid, and gas we see is made of billions of these tiny particles.
Teacher's Tip: Think of an atom as the "LEGO" of the universe.
Exam Tip: Mention that an atom contains subatomic particles to show modern understanding.

Question 2: Describe a discharge tube.
Answer: A discharge tube is a cylindrical glass tube in which a gas can be filled and which is fitted with metal plates called electrodes at each end.
This tool was essential for discovering electrons and protons. When high voltage is applied, the gas inside glows or emits rays.
Teacher's Tip: It's similar to the neon tubes you see in bright signs.
Exam Tip: Use the word "electrodes" and specify their location at "each end."

Question 3: What is the central part of an atom known as?
Answer: Nucleus consisting of neutrons and protons.
The nucleus is incredibly tiny compared to the whole atom, but it contains almost all of its weight. It is always positively charged because of the protons.
Teacher's Tip: The Nucleus is the "Heavy Heart" of the atom.
Exam Tip: Always mention both protons and neutrons as the components of the nucleus.

Question 4: What do you mean by electron transition?
Answer: The jumping of an electron from one energy level to another is called electronic transition.
This happens when an atom gains or loses energy. When an electron jumps back down, it often releases energy as a flash of light.
Teacher's Tip: Think of it as moving between floors in a building.
Exam Tip: Mention "energy levels" or "shells" to explain the start and end of the transition.

Question 5: Define an ionic compound.
Answer: An ionic compound is defined as the compound formed by the oppositely charged ions which attract each other to form a chemical bond e.g. Sodium chloride.
This bond forms when one atom gives an electron to another. The positive and negative ions then stick together like magnets.
Teacher's Tip: Ionic bonds are like magnetic attraction - opposites attract!
Exam Tip: Always provide a standard example like NaCl (Sodium chloride) for definitions.

Question 6: Define the atomic mass number of an atom.
Answer: The atomic mass number refers to total number of protons and neutrons in a nucleus.
Since electrons weigh almost nothing, we don't count them in the mass number. It tells us how heavy the core of the atom is.
Teacher's Tip: Mass = Matter in the core (p + n).
Exam Tip: Do not confuse atomic number (protons only) with mass number (protons + neutrons).

Question 7: Name the isotopes of hydrogen.
Answer: There are 3 (three) isotopes of hydrogen:
1. Protium ₁H¹
2. Deuterium ₁H²
3. Tritium ₁H³
These atoms all act like hydrogen but have different weights because of extra neutrons. Tritium is the heaviest and is actually radioactive.
Teacher's Tip: Memorize the sequence: 1, 2, 3 mass.
Exam Tip: Write the symbols (₁H¹, etc.) along with the names for a better answer.

Question 8: What is radioactivity?
Answer: Radioactivity is the phenomena in which certain substances emit radiations spontaneously.
This happens when an atom's nucleus is unstable and starts "breaking" or releasing energy. It occurs naturally in elements like Uranium.
Teacher's Tip: Think of it as a "leaky" nucleus releasing energy rays.
Exam Tip: Use the word "spontaneously" as it means the radiation happens on its own.

Question 9: Name three inert gases.
Answer: Inert gas: Argon, Helium, Neon.
These gases are called "inert" or "noble" because they don't like to mix or react with other elements. Their outer electron shells are already full and happy.
Teacher's Tip: Noble gases are "antisocial" - they don't want to bond!
Exam Tip: Capitalize the names of the gases as they are proper chemical names.

Question 10: What is the charge on a neutron?
Answer: No charge.
Neutrons are perfectly neutral, which is how they got their name. Their main job is to act as "glue" in the nucleus between the positive protons.
Teacher's Tip: Neutron = Neutral = Zero charge.
Exam Tip: Simply writing "0" or "No charge" is sufficient, but ensure clarity.

Question 11: What is the number of neutrons present in a potassium atom?
Answer: 20.
Since potassium has a mass of 39 and atomic number 19, we subtract: 39 - 19 = 20. This specific count helps identify that specific isotope of potassium.
Teacher's Tip: Subtract the smaller number from the larger number to get neutrons.
Exam Tip: Always verify the atomic mass and number from the periodic table data given.

ANSWER IN DETAIL

Question 1: State the postulates of the Dalton’s atomic theory.
Answer: Postulates of Dalton’s atomic theory:
1. All matter is made up of minute particles called atoms.
2. Atom are indivisible. They can neither be created nor destroyed.
3. All atoms of a given element are identical in mass and size.
4. The atoms of one element are different from the atoms of other elements.
5. During a chemical reaction, the atoms of an element combine with atoms of another element in a fixed ratio.
Dalton imagined atoms as tiny, hard, indestructible billiard balls. While we've discovered subatomic particles since then, his idea of "fixed ratios" remains the foundation of chemistry.
Teacher's Tip: Dalton = Indivisible Ball theory.
Exam Tip: Number your points clearly to help the examiner read them easily.

Question 2: Name the three subatomic particles. How are they different from each other?
Answer: The three subatomic particles are:
1. Electron: It is represented by e^- it has a unit negative charge and possesses mass equal to 1/1837 times that of a proton.
2. Proton: It is represented by ₊₁p¹. It has a unit positive charge. Its mass is equal to the mass of a hydrogen atom.
3. Neutron: It is denoted as ₀n¹. It has no charge on it. Its mass is almost equal to the mass of hydrogen atom.
The big differences are charge and weight. Protons and neutrons are heavy and live in the core, while electrons are feather-light and zip around the outside.
Teacher's Tip: Protons and Neutrons are "Heavies"; Electrons are "Lights."
Exam Tip: Always mention the fractional mass (1/1837) for electrons to show accuracy.

Question 3: Enlist the features of Bohr’s model of an atom. How did he correct the Rutherford’s model?
Answer:
Bohr’s Postulates:
1. The extra-nuclear region of an atom consists of electrons revolving in particular orbits called shells.
2. Each shell has a fixed energy level.
3. As long as an electron revolves in the same shell, the energy remains constant. During electronic transition energy is either released or absorbed.
Bohr corrected Rutherford’s model by suggesting the concepts of energy levels and electronic transition.
Bohr fixed the "crash" problem in Rutherford's model by saying electrons have fixed seats (shells). He explained that electrons only change energy when they jump between these seats.
Teacher's Tip: Bohr = Energy Shells (seats) for electrons.
Exam Tip: Use the word "stationary" or "fixed" energy levels when describing his shells.

Question 4: What is the maximum number of electrons that L shell can accommodate? Give reasons for your answer.
Answer:
The maximum possible number of electrons that a shell can accommodate can be given by the formula 2n² (Here n is the number of shell). In case of L shell n = 2. Therefore 2n² = 2(2)² = 8. Thus L shell can accommodate maximum number of 8 electrons.
This formula (2n²) prevents too many electrons from crowding into one layer. As we move out from the center, the shells get larger and can hold more "passengers."
Teacher's Tip: K=1, L=2, M=3... alphabetical order starting with K!
Exam Tip: Always write the calculation 2 × 4 = 8 to show how you got the answer.

Question 5: Draw the electronic configuration of the following elements clearly stating the number of neutrons and protons in their nucleus. (a) Beryllium (b) Silicon (c) Chlorine (d) Aluminium (e) Nitrogen
Answer: (Diagrams for each):
a. Beryllium: Core (4p, 5n), Shells (2, 2)
b. Silicon: Core (14p, 14n), Shells (2, 8, 4)
c. Chlorine: Core (17p, 18n), Shells (2, 8, 7)
d. Aluminium: Core (13p, 14n), Shells (2, 8, 3)
e. Nitrogen: Core (7p, 7n), Shells (2, 5)
These pictures show the internal "apartment building" of the atom. Protons and neutrons are in the "lobby" (nucleus), and electrons live in the "floors" (shells) according to their numbers.
Teacher's Tip: Always fill the first ring with 2 before putting any in the second ring.
Exam Tip: Draw your electron dots on the shell lines, not in the empty space between them.

Question 6: How are isobars different from isotopes?
Answer: Isotopes are defined as elements which have the same atomic number but different mass number e.g. hydrogen has isotopes namely: Protium ₁H¹, Deuterium ₁H², Tritium ₁H³.
Isobars are the elements which have same mass number but different atomic number e.g. potassium and argon are isobars.
Isotopes are the same element with different weights (like siblings). Isobars are different elements that happen to weigh the same (like two different fruits that both weigh exactly 100 grams).
Teacher's Tip: Iso-TOPES = same element; Iso-BARS = same weight (Bar = weight).
Exam Tip: Use a comparison table to show same vs. different for atomic and mass numbers.

Question 7: Write a note on valence shell.
Answer: The outermost shell of an atom is called its valence shell. The number of electrons that an atom needs to accept, donate or share so as to form a complete outer octet (eight electrons) is called the valency of that element. So, the valency of an element can be defined as its combining capacity with other elements. The valency of an element is always a whole number.
The valence shell is the most important part of an atom for chemistry. It's like the atom's "hands" - it uses the electrons here to hold onto other atoms and form molecules.
Teacher's Tip: Valency = The number of "hands" an atom uses to hold others.
Exam Tip: Mention that valency is a "whole number" as it refers to counting real electrons.

Question 8: What is nuclear fission? How is it different from nuclear fusion?
Answer: 
Nuclear fission: The process in which a heavy nucleus, when bombarded with slow moving neutrons, splits into two or more lighter nuclei accompanied with a release of tremendous amount of energy.
Nuclear fusion: The process in which two light nuclei combine together at very high temperature, and form a stable and a heavier nucleus and is accompanied with a release of tremendous amount of energy.
Fission is "splitting apart," like a glass breaking. Fusion is "joining together," like two drops of water merging into one large drop. Both release huge amounts of power.
Teacher's Tip: Fission = Divide; Fusion = Fuse (Join).
Exam Tip: Highlight "high temperature" as a specific requirement for fusion to occur.

Question 9: What are the various components of a nuclear power plant?
Answer: Nuclear power plant has the enlisted components.
1. Fuel: Uranium - 235 or plutonium - 239 which undergo fission easily are used as fuels.
2. Moderator: Heavy water or rods of graphite are used as moderators. The controlling rods made of cadmium or boron steel are used to regulate the number of neutrons in the reactor.
3. Coolant: As a great amount of heat is released during this process, a coolant is also required. Liquid sodium and heavy water are generally used as coolants.
A nuclear plant uses atomic splitting to create heat, just like a coal plant uses fire. The "Moderator" keeps the reaction safe, and the "Coolant" carries the heat away to make electricity.
Teacher's Tip: Think of Uranium as the "Battery" and Coolant as the "Radiator."
Exam Tip: List all three (Fuel, Moderator, Coolant) with their specific materials (e.g. Uranium, Graphite, Liquid Sodium).

Question 10: How are radioactive radiations useful in medicines?
Answer: Controlled radiation is used during treatment of cancer, as they can kill cancer cells. Cobalt-60 is used for destroying malignant cells in patients suffering from cancer. Iodine-131 is used for studying disorders of thyroid gland. Sodium-24 is used for examining the circulation of blood.
Radiation can be a "targeted weapon" against sickness. Because these rays can be tracked, doctors use them like tiny flashlights to see inside our bodies or kill dangerous cells.
Teacher's Tip: Radiation is a "Double-Edged Sword" - dangerous, but can cure if used carefully.
Exam Tip: Memorize the specific isotope names (Cobalt-60, Iodine-131) and their exact medical use.